Jackup barges having three legs are customarily used in erecting temporary drilling platforms above the ocean at offshore locations. An offshore drilling barge must be positioned substantially above the level of the water, even higher than the wave action anticipated in a violent storm to enable offshore drilling. The platform normally weighs several million pounds. Normally, it is substantially large typically measuring as much as two hundred feet along a side. The platform is supported on three legs. The legs extend from the platform to a bottom plate which enables them to rest on the mud beneath the body of water. An elevating mechanism lifts the platform on the three legs at the time of elevation. Typically, the barge is towed with the legs raised and the platform floating on the water. At the time of erection, the legs are lowered until they encounter the bottom, and they support the platform as it is raised above the water.
The legs are quite sizable. On a modern rig, the legs may be as much as four or five hundred feet tall. The legs typically are formed of a triangular open lattice work which is as much as fifty feet across. The triangular shape is open and is typically formed of structural members as large as four or five feet thick. Various and sundry types of structural members are used in the construction of jackup legs. Box beams, I beams, flanged plates, and tubular members have been used all to form various and sundry jackup legs. Jackup legs typically must have horizontal beams to reinforce the three corner members. In addition they normally include reinforcing members at angles extending from corner to corner to give rigidity to the entire leg structure. The horizontal beams and the angularly positioned reinforcing members basically tie the three corner members together, thereby defining a structure which is able to support the weight of the drilling barge.
It is necessary for the platform to include equipment which lifts the platform on the jackup legs. Many types of equipment have been suggested including devices which insert pins into holes in the legs, rack and pinion mechanisms and the like. The leg thus must necessarily be constructed with a mechanism extending along its length for engagement with the lifting mechanism and this typically includes at least a set of holes or a rack. Typically, the rack or the holes must be aligned with the mechanism on the platform and to this end, appropriate flanged plates or alignment members are incorporated. The rack and alignment members are typical devices known previously are necessary.
Constructions used in the past have further required additional strength in that they have to be formed with exposed racks and appropriate alignment flanges. While these may impart vertical strength and otherwise serve as support columns enabling the platform to be raised on the leg, they have an unfavorably shaped profile presented to the water. The profile of the leg in the water is very important. Ordinarily, the leg is an open lattice work. This enables waves to pass through the leg. Nevertheless, the leg is not simply transparent to wave action. The tendancy of the leg to bend under wave impact is proportional to a factor determined by the streamlining of the leg as viewed from any direction. It is not possible to know in advance the direction from where waves originate. Accordingly, the three legs of a jackup barge must all be designed to have a required resistance to bending in all directions. That is to say, it is not possible to know in advance the direction of the wave action and additionally to know the precise azimuthal positioning of the drilling rig relative to the wave action. Accordingly, the leg of the drilling rig must all resist bending in all directions around the compass. Further, the profile of the leg or the streamlining as viewed in any particularly direction must be made optimum. The most optimum arrangement is a circular frame member at each corner of the leg. This construction presents the minimum resistance to wave action without regard to the direction of impingement. However, it is well neigh impossible to use a circular frame member because the leg must incorporate a rack and appropriate guides to enable the platform elevation equipment to operate. Some jack-up rig designs include vertical racks attached to legs made of circular stock. The rack presents an irregularity in shape to the wave action and detracts from the streamlined shape of the circular leg. A circular leg is difficult to use with as a guide because it does not provide an edge for grasping. If the guide fits around the entire leg, the leg must be built to rather expensive tolerances. Accordingly, the present invention has been devised.
This device enables the use of circular stock in the three corners of the triangular legs. This reduces the profile of the completed legs to a minimum, thereby reducing the bending which occurs in wave action and further increasing the strength of the leg. This results in a reduction of metal in the leg. In a representative leg for a full size drilling rig, the leg might weigh as much as six million pounds if constructed in accordance with teachings of the prior art. Incorporating the leg construction of this disclosure, a reduction of one million pounds or more of metal in the leg is possible. The reduction of bending moment reduces the design criteria in the hull itself, thereby reducing the weight and cost of the hull. The overturning moment is also reduced which allows closer spacing of the legs of a smaller hull. This reduces the cost of construction, increases the speed of towing, increases the bending resistance, and otherwise provides a more efficient platform design. As a consequence, the rack for cooperation with the elevation equipment and the guides which provides positive engagement therewith are recessed in the circular cross section of the corner members of the leg to thereby provide an optimum reduction in profile.